Method and apparatus for determining a vapor signature based upon frequency

ABSTRACT

An apparatus for sensing analyte is provided. The apparatus may include a plurality of sensor array columns, each sensor array column including a plurality of sensors, each sensor being adapted for sensing one or more parameters in reaction to the presence of one or more analytes and output a current therefrom in accordance with level of the sensed parameter, a Voltage Controlled Oscillator (VCO) array including a plurality of VCOs, each VCO adapted to receive an output current from one of the plurality of sensors from each of the plurality of sensor arrays and for and generating an output oscillation frequency in accordance with the level of the received output current, and a counter array including a plurality of counters, each counter adapted to receive an output from a corresponding VCO and count a number of oscillations over a predetermined time.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/330,414, filed May 3, 2010 to Dimmler et al. andtitled METHOD AND APPARATUS FOR SENSING GASSES; U.S. Provisional PatentApplication Ser. No. 61/330,415, filed May 3, 2010 to Dimmler et al. andtitled METHOD FOR SENSING GASSES; and U.S. Patent Application Ser. No.61/330,416, filed May 3, 2010 to Dimmler et al. and titled METHOD ANDAPPARATUS FOR SENSING GASSES, the entire contents of these threeprovisional patent applications being incorporated herein.

FIELD OF THE INVENTION

The present invention relates generally to the sensing of gasses, and inparticular to the sensing of one or more gasses utilizing one or morehybrid composite silicon/organic field effect devices.

BACKGROUND OF THE INVENTION

There are a wide variety of sensor technologies available. While many ofthese technologies may be used to sense one or more gasses, each comeswith one or more drawbacks or tradeoffs as compared with the presentinvention, most are not suitable for wide-spread portable applicationsfor a variety of reasons. Many sensors are physically too large forportability. Others consume too much power to be reasonably operated bya battery. Others operate at very high temperatures making themdifficult to use in portable applications. Still others are tooexpensive for wide-spread use, and many require training to useproperly. A survey of existing mainstream sensor technology is outlinedbelow.

Chromatography

A chromatograph is an apparatus that separates a complex mixture intoindividual components. A mixture of gas components is injected into achromatograph column, where the components travel down the column atdifferent rates, reaching the end of the column at different times. Adetector is positioned at the end of the column to quantify theconcentrations of individual components of the mixture as they reach theend of the column.

The apparatus used in gas chromatography consists of four basiccomponents: (1) a carrier-gas supply and flow controller, (2) a sampleinlet system providing a means for introduction of the sample, (3) thechromatographic column and associated column oven, and (4) the detectorsystem. The column is the heart of the gas chromatograph. Typically thecolumn is a glass or metal tube of that is 6 to 13 mm in diameter and 1to 2 meters in length. The chromatic column is filled with a carriergas, most commonly nitrogen or helium, though other gases such as carbondioxide, argon, xenon, and hydrogen are occasionally used.

The detector produces a response that is proportional to component thatis separated by column and is located at the end of the column.Different detectors may be utilized dependent upon the analyte ofinterest. Any one of many different types of detectors could be used,such as an ion mobility spectrometer (IMS), mass spectrometer, photoionization detector (PID), flame ionization detector (FID), thermalconductivity detector (TCD), electron capture detector (ECD), flamephotometric detector (FPD) or far UV absorbance detector (FUV).

Chromatography is a well known and accurate technique for identifyingconstituents in a gas mixture. However, these units tend to be large andexpensive, and specialized training is required to use them effectively.

Ion Mobility Spectrometry (IMS)

The gaseous sample to be analyzed enters a spectrometer, where it isionized by a radioactive source. The resulting positive and negativecharged species are then accelerated over a short distance and thetime-of-flight is determined. The IMS differs from mass spectrometer inthat it operates under atmospheric conditions and does not need largeand expensive vacuum pumps. Because of this, IMSs can be miniaturized.

Like Chromatography, IMS is well know and accurate. However, IMS unitsare large and expensive, and require trained personnel to make ameasurement.

Mass Spectrometry

The principle of the mass spectrometer is similar to the ion mobilityspectrometer, except a vacuum is required. The sampled gas mixtures areionized, and charged molecular fragments are produced. These fragmentsare sorted in a mass filter according to their mass to charge ratio. Theions are detected as electrical signals with an electron multiplier or aFaraday plate. Low mass ions are displayed as a vertical line at theleft end of a scale while heavy ions are displayed towards the right.The length of a line represents the quantity of that ion in the gasmixture.

Mass Spectrometry is well know and accurate. However, IMS units arelarge and expensive, and require trained personnel to make ameasurement.

Electrochemical Sensors

There are a wide variety of electrochemical sensors, which can beclassified in two basic groups called potentiometric, in which a voltagemeasured and amperometric, in which a current is measured. These sensorsconsist of a casing that contains a collection of chemical reactants(electrolytes or gels) in contact with the surroundings through twoterminals, an anode and a cathode. For gas sensors, the top of thecasing has a membrane which can be permeated by the gas sample.

The presence of the gas to be detected causes oxidization to take placeat the anode and reduction to take place at the cathode. A current iscreated as the positive ions flow to the cathode and the negative ionsflow to the anode. Gases such as oxygen, nitrogen oxides, and chlorine,which are electrochemically reducible, are sensed at the cathode whileelectrochemically oxidizable gases such as carbon monoxide, nitrogendioxide, and hydrogen sulfide are sensed at the anode. Potentiometricmeasurements are performed under conditions of near-zero current.Amperometric sensors are usually operated by imposing an external cellvoltage sufficiently high to maintain a zero oxygen concentration at thecathodic surface. Sensitivity of amperometric sensors is generallybetter than potentiometric sensors.

Electrochemical sensors are wide-spread, but are typically too large andrequire too much power for portable applications. In addition, thesesensors typically utilize toxic materials that are not suitable forportable applications.

Fiber Optic Sensors

Fiber optic sensors are a class of sensors that use optical fibers todetect chemical contaminants. Light is generated by a light source andis sent through an optical fiber. The light then returns through theoptical fiber and is captured by a photo detector. Some optical fibersensors use a single optical fiber while others use separate opticalfibers for the light source and for the detector.

There are three general classes of fiber optic sensors The first type iscompletely passive. A spectroscopic method can be used to detectindividual types of contaminants. This method involves sending a lightsource directly through the optical fiber and analyzing the light thatis reflected or emitted by the contaminant. A second class of fiberoptic sensors consist of a fiber optic sensor with a chemicallyinteracting thin film attached to the tip. This film is formulated tobind with certain types of chemicals. Contaminant concentration can befound by measuring the color of the thin film, the change in refractiveindex, or by measuring the fluorescing of the film. The third type offiber optic sensors involves injecting a reagent near the sensor. Thisreagent reacts either chemically or biologically with the contaminant.The reaction products are detected to give an estimate of thecontaminant concentration.

These types of sensors have the advantage of power. Several types haveno moving parts. They are also capable of detecting various chemicals atvery low concentrations. However, they are limited by the ability totransmit light through the optical fiber over long distances. Someorganic pollutants are not easily differentiated using UV-visiblespectroscopy.

Colorimetry

Colorimetric devices work by analyzing the color of contaminated waterthat has been mixed with a particular chemical reagent. Pre-measured,unit-dose reagent that react with water samples are available for sale.To test water samples, the pocket colorimeter compares a reacted samplewith a sample blank and yields results in concentration units.

Pocket colorimeter test kits are portable and are simple to use. Theyprovide visual evidence of a gas detection event, and they are not proneto interferences. However, these devices have limited chemicalsensitivity to individual VOCs. They also need actual water samples fortesting, and cannot therefore be used in situ. Most kits do not meetU.S. EPA method requirements and may not be used for compliancemonitoring.

Infrared Sensors

Infrared sensors can be used to detect gases, which, in general, haveunique infrared absorption signatures in the 2-14 um range. Theuniqueness of the gas absorption spectra enables identification andquantification of chemicals in liquid and gas mixtures with littleinterference from other gases. These devices are typically comprised ofa source of infrared radiation, a detector capable of seeing theinfrared radiation, and a path between the detector and source that isexposed to the gas being detected. When gas in the path absorbs energyfrom the source, the detector receives less radiation than without thegas present, and the detector can quantify the difference.

Mass Sensor—Surface Acoustic Wave Sensors/Portable Acoustic Wave Sensors

Surface Acoustic Wave Sensors (SAWS) are small miniature sensors used todetect VOCs. A SAW device consists of an input transducer, a chemicalabsorbent film, and an output transducer on a piezoelectric substrate.The piezoelectric substrate is typically quartz. The input transducerlaunches an acoustic wave which travels through the chemical film and isdetected by the output transducer. The device runs at a very highfrequency, generally about 100 MHz. The velocity and attenuation of thesignal are sensitive to the viscoelasticity as well as the mass of thethin film which can allow for the identification of the contaminant.Heating elements under the chemical film can also be used to desorbchemicals from the device. A signal pattern recognition system that usesa clustering technique is needed to identify various chemicals.

SAWS have been able to distinguish organophosphates, chlorinatedhydrocarbons, ketones, alcohols, aromatic hydrocarbons, saturatedhydrocarbons, and water. They are small, low power, have no moving partsother than the high-frequency excitation, and are known to have goodsensitivity to various chemicals, able to detect chemicals in very lowconcentrations. However, they are generally not able to discriminateamong unknown mixtures of chemicals.

Metal-Oxide Semiconductor Sensors

Metal oxide sensors are the among the earliest and most popular sensorelement in sensor arrays. For example, oxygen sensors used in everyautomobile are metal oxide sensors. A metal oxide sensor is an n-typeinorganic semiconductor, such as tin oxide, doped tin oxide derivatives,zinc oxide, or iron oxide. These materials are heated to temperatures of300 C to 550 C and used as a two-terminal resistive device. The responsearises from the reduction of the gas species at the surface of thesemiconductor which increases the electron carrier concentration,resulting in higher conductivity. The gas species is consumed as thesensor operates.

The sensor is traditionally constructed using a ceramic support tubecontaining a platinum heater coil and the metal oxide is coated onto theoutside of the ceramic tube with the appropriate electrical leadsconnected to the film. In recent decades, newer implementations of themetal oxide sensor have been constructed with planar configurationsusing conventional micro-fabrication techniques. While usingmicro-fabrication methods is advantageous, there is a significantchallenge with integration of multiple sensor elements into a singlearray. The procedure can be difficult and expensive due to extensivesubtractive processes arising from materials compatibility across thedifferent elements. In addition, there are complicating issues relatedto its high operating temperature.

Another disadvantage of the metal oxide is power consumption. Becausethe sensing response depends directly on the reduction reaction, whichis very sensitive to the temperature, the sensor must be heated to atleast 300 C. Generating such high temperatures is consumption, typicallyaround 800 mW. Yet another disadvantage is a general lack ofdiscrimination since the combustion mechanism limits the sensitivity ofthe device and is not reliant on the chemistry of the gas speciesitself.

Despite the these disadvantages of power consumption and integration,the metal oxide is by far the most commonly used gas sensor incommercially available electronic noses. The sensor is rugged and ratherversatile and is already used in many other industrial settings. Ofthose that are often used for industrial applications, the tin oxidesensor doped with palladium or platinum is the most popular. However,the use of metal oxides continues to pose a fundamental problem ofdifficult integration.

Polymer-Absorption Chemiresistors

The concept of using polymeric absorption to detect the presence ofchemicals in the vapor phase has existed for several decades. Thesepolymer-absorption sensors, also called chemiresistors, consist of achemically sensitive absorbent that is deposited onto a solid phase thatacts as an electrode. When chemical vapors come into contact with theabsorbent, the chemicals absorb into the polymers, causing them toswell. The swelling changes the resistance of the electrode, which canbe measured and recorded. The amount of swelling corresponds to theconcentration of the chemical vapor in contact with the absorbent. Theprocess is reversible, but some hysteresis can occur when exposed tohigh concentrations. Several companies and organizations have developedchemiresistors, but the specific attributes and types of absorbents,which are generally proprietary, vary among the different applications.

Chemiresistors are attractive because they are small, low power devicesthat have no moving parts and have good sensitivity to variouschemicals. However, these types of sensor generally react to a broadarray of analytes, making it difficult to identify a specific gas. Somepolymers react strongly to water vapor and moisture. Although thereaction with the analyte is designed to be reversible, the signal mayexperience hysteresis and a shift in the baseline over time when exposedto chemicals.

Conducting Polymer Sensors

A conducting polymer sensor is based on a polymer material possessingelectrical properties that can selectively absorb specific odorants.Exposure to a gas analyte induces changes in the electrical behavior ofthe sensor. The sensor response arises from intermolecular interactionsbetween the sensor and the analyte. These interactions are the result ofhydrogen bonding, dipole-dipole or dipole-induced dipole dispersions,and hydrophobic forces. For this reason, conducting polymer sensor arrayare considered the most similar to the olfactory sensor of a biologicalnose.

The most popular materials is polypyrrole, a key feature of the polymerof which is the repeating and alternating arrangement of double bondsthroughout the molecule. This feature, known as a conjugated pi electronsystem, gives rise to the electrical behavior. Reduction and oxidationof the polymer can subsequently modify its charge conducting behaviorand it is surmised that the sensor response may result from reductionand oxidation processes or similar interactions involving partial chargetransfer. Polypyrrole is most often deposited using electrochemicalpolymerization in order to avoid solvent compatibility issues. Theproperties of the film are strongly affected by the growth conditionsbut with careful control, purified and reproducible films can beachieved.

Conducting polymer sensors have several advantages. In general, theyrespond to a broad range of organic vapors and there is a large spectrumof materials that can be synthesized allowing for a wide range ofselectivity. There are synthesized using relatively low cost materialsand the sensor element can be fabricated with an attractive form factor.Unlike metal oxide sensor, conducting polymer sensors operate at roomtemperature, reducing power consumption and providing for longerlifetime.

A major disadvantage of a conducting polymer sensor is they tend to besensitive to humidity and show a long-term drift in their performance.Further, they do not generally display a high specificity to individualgases. However, these polymers can be chemically tailored to enhancedifferences to response to classes of molecules.

ChemFET Sensors

A Chemically sensitive FET, also known as a ChemFET, is the structuralanalog of a MOSFET, with the polysilicon gate replaced with a chemicallysensitive layer. The gate is chosen such it reacts with the gaseousspecies to cause a work function shift that can be detected through FEToperation. Though ChemFETs are amenable to CMOS integration, in practicefabrication is greatly complicated since most polymers areelectrochemically deposited.

Therefore, it would be desirable to provide a sensor that overcomes thedrawbacks of the prior art to make it suitable for low-cost, low-power,wide-spread, and portable applications.

SUMMARY OF THE INVENTION

A key attribute to reducing the cost of sensor technology is integrationinto a standard silicon CMOS process. Traditional sensor technologies,however, are not conducive to direct implementation directly on silicon,due to material incompatibility, deposition incompatibility, oroperating temperature.

As can be seen from the description of the sensor technologies describedabove, the technology that lends itself best to simple integration intoa CMOS silicon fabrication process are the sensors based on polymers.For this reason, polymer-based sensors are among the most promisingtechnologies for an entry into high-volume, low-cost sensor markets.However, as discussed above, these sensor technologies have limitations.The polymer technology as set forth in accordance with the presentinvention addresses these shortcomings

One or more embodiments of the invention described herein may beimplemented employing improvements to a sensor cell as described in U.S.Pat. No. 7,397,072 (and as is shown in FIG. 1), and a method ofimplementation of that improved cell. This improved cell structureallows measurement of two electrical signals, namely the current throughthe silicon transistor (a current reading between ss 208 and sd 206 inFIG. 2) and the polymer current (a current reading between os 204 and od203 in FIG. 2). Of course, other sensor cells that may include similarelectrode positioning may be employed in accordance with the inventionas appropriate and desired. Each of these two currents represents adifferent analyte characteristics. These currents are preferablymeasured at the same instant of time, and therefore are differentrepresentations of same analyte concentration of the measured gas.

The silicon current is caused by minority carriers in the silicon 210that are attracted to charge trapped in the polymer, thereby changingthe threshold of the MOS device. This change of threshold gives rise toa change in current at any given source/drain voltage.

The change in current through the organic material is a measured byelectrodes, which are in contact with the organic polymer. The resultingpolymer current (Ios 204-Iod 203) is representative of a change inresistance of the polymer due to analyte interaction. As will be seenlater, these two currents about the analyte will become part of the“vapor signature” of the vapor, including an analyte, being detected.The fact that this sensor cell offers two different currents torepresent a given analyte enhances the uniqueness of the vapor signaturefor a given analyte, and will translate to simpler post-measurementanalysis to identify the analyte.

The Sensor Cell Array:

The precise value of these currents will depend on how much analyteinteracts with the polymer in the sensor cell. This interaction can becontrolled for any given cell by adding certain receptor molecules, theinteraction with a given analyte thereby being increased or otherwisemodified. A given receptor molecules are preferably designed to attracta certain class of chemicals. The sensing materials are organicsemiconductor polymers doped with a small molecule receptor, but canalso be other types of polymers such as insulator polymers with carbonnanoparticles. All sensor materials used are solution-based, allowinglow-cost deposition methodologies such as inkjet printing.

Therefore, in accordance with the present invention, a plurality ofthese receptor-doped sensor cells, each with a different receptormolecule, or different levels of a single molecule, may be placed intoan array. Applied to such an array, each component of a vapor beingmeasured will give rise to a different silicon and polymer current fromeach of the receptor-doped sensor cells of the array, the accumulationof which make up an “electronic signature” of the vapor. This electronicsignature can then be compared to previous measurements to identify thesame vapor or components of the same vapor.

Analyte Control in the Sensor Cell:

Heating of a receptor in, for example, a sensor cell of the presentinvention, will cause the analyte and the receptor to dissociate. Forthis reason, in many traditional sensor systems the sensor is placed ona heating pad. After a given measurement is complete, heat is applied tothe sensor, releasing the analyte. Gas is also purged away from thesensor surface. The cell is then now ready for another measurement. Suchmechanisms around the sensor significantly raise the cost of the sensorsystem, and also require time between measurements, thus reducing thenumber of tests that may be performed in a fixed period of time. Thesensor cell incorporated into the present invention, on the other hand,offers several advantages in controlling the analyte introduction intothe cell that come about due to the ability to control the electricfield in the polymer.

Due to the structure of the inventive sensor cell, the vertical electricfield through the polymer can be controlled. When the electric fieldinduces a positive charge on the polymer, the analyte is absorbed intothe polymer. When the electric field is reversed, the analyte isrepelled from the polymer. Therefore, the control of the electric fieldin the sensing polymer of the cell reduces the need for valves andpiping systems to bring the analyte to and from the sensing surface.

Since these cells are preferably built directly into the siliconsubstrate, it is possible to utilize an n+ diffusion runner in p-typesubstrate or a p+ diffusion running in n-type substrate to act as alocalized heating source. Current running through the diffusion willgenerate sufficient heat to further dissociate the analyte and thereceptor in the localized area while reducing overall hardware cost andtime for heating.

Sensor Cell Array Architecture with Interchangeable Reference Cell:

The attributes of the inventive sensor cell described above lend itselfto the inventive array architecture constructed in accordance with analternative embodiment of the invention that results in substantialtechnical advantages over other cell systems. A diagram of this arrayarchitecture is shown in FIG. 4, and will be described in greater detailbelow.

In accordance with this embodiment of the invention, the sensor arraypreferably consists of two or more substantially identical columns ofsensor cells, each column consisting of individual cells with a polymersensor sensitive to different analytes. All or some of the sensor cellsof one or more of the columns may be heated by the localized heatingmethod described above. One or more columns of sensors are preferablynot heated. The collection of all silicon and polymer cell currents ineach column or array may generate a “vapor signature”, but vaporsignatures of each column may differ in response to differences in thesensor temperature. The comparison of the vapor signatures at thevarious heated and unheated columns and/or cells thereof may then becompared for the following purposes:

Continuously Updated Baseline:

At elevated temperatures, the analyte and the receptor are far lesslikely to bond. Therefore, the current generated in the heated sensorscan act as a baseline in comparison to the sensor cells at roomtemperature. In traditional sensor arrays, the baseline is generated bya measurement made by the same sensor at a previous time. In thisembodiment of the present invention, the baseline is continuouslyupdated at the very same time as the analyte is measured with theunheated sensor, giving rise to several benefits:

(1) The measurement will be more accurate since the baseline is measuredat the same time as the analyte measurement, eliminating errors createddue to the time shift between the baseline measurement and the analytemeasurement.

(2) Automated calibration to moisture.

(3) The analyte measurements can be taken at more frequent intervals, oreven continuously, since there is no need to stop the sensor measurementto re-establish a baseline.

Activation Energy Information:

Activation energy is a measure of the temperature dependence of achemical reaction. Specifically, when ln(x) vs 1/T is plotted, the slopeof that equation is the activation energy Ea. This information can beobtained for both the silicon device and the organic device of thesensor cells. Since activation energy is specific to the type ofanalyte, this information helps significantly in identifying the analytewith reduced signal analysis.

Reduction of Signal Noise Due to Moisture:

Polymer materials are known to pick up moisture over time, changing thebaseline signal. This change in baseline signal will add onto apreviously measured vapor signature, giving rise to a different level.The moisture issue is traditionally resolved by periodically heating thesensor cells. During this heating time, the sensor array istraditionally unable to make any measurements. In the case of theinventive array architecture, however, moisture is removed byperiodically changing the heated and unheated column.

Thus, in accordance with the invention, the proposed electronic noseconsists of an array of sensors, each of which will yield a “signature”of the analyte being sensed at any given period of time by means of acurrent level. A series of such current level measurements mayultimately be used to identify a particular analyte. Current levels,however, are difficult to read with currently available digitalcircuitry, and are even harder to store. The present invention overcomesthese drawbacks. Therefore, in accordance with the present invention, animproved method and apparatus for sensing gasses is provided employingan array structure of receptor cells to provide a vapor signature forone or more tested gasses.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification anddrawings.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combinations of elementsand arrangement of parts that are adapted to affect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference is made tothe following description and accompanying drawings, in which:

FIG. 1 is cross section of a prior art four-terminal FET chemical sensordevice as described in U.S. Pat. No. 7,397,072;

FIG. 2 is a cross section of an improved cell structure in accordancewith an embodiment of the invention;

FIG. 3 is a sensor array constructed in accordance with an embodiment ofthe present invention;

FIG. 4 is a timing diagram depicting an inventive sequence ofmeasurements in accordance with an embodiment of the invention;

FIG. 5 is a sensor array constructed in accordance with an alternativeembodiment of the invention;

FIG. 6 is a layout of a cell heater in accordance with the invention;and

FIG. 7 is a sensor array constructed in accordance with an additionalalternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described making reference to the followingdrawings in which like reference numbers denote like structure.Referring to FIG. 2, an illustration of a cell structure that improvesover prior art U.S. Pat. No. 7,397,072, and is constructed in accordancewith an embodiment of the present invention, is shown. As is shown inFIG. 2, N+ regions 201 and 202 extend underneath the P+ Poly Sielectrodes 203 and 204 and above silicon wafer 209. Such placement ofthe N+ regions provides an advantage of a channel underneath the P+ polysilicon electrodes 203 and 204 regardless of the potential of thoseelectrodes. This inventive structure allows measurement of twoelectrical signals, namely the current through the silicon transistor(denoted “I(SS)-I(SD)”, and comprising a measure of current flow betweenthe SS electrode 208 and the SD electrode 206) and the current throughthe polymer (denoted “I(OS)-I(OD)”, and comprising a measure of thecurrent flow between the OS electrode 204 and the OD electrode 203.

Each of these two currents represents a different analytecharacteristic. These currents are preferably measured at the sameinstant of time, and therefore comprise different representations of asame analyte concentration of a measured gas. The silicon current flow(the current measured between SS 208 and SD 206) is caused by minoritycarriers in the silicon that are attracted to charge trapped in thepolymer, thereby changing the threshold of the MOS device. This changeof threshold gives rise to a change in current at any given source/drainvoltage. The change in current through the organic material is measuredby electrodes 203 and 204, which are in contact with the organic polymer207. The current through the polymer results from a change in resistanceof the polymer due to analyte interaction. As will be described below,these two currents will become part of the “vapor signature” of thevapor being detected. Providing two different currents to represent agiven analyte enhances the uniqueness of the vapor signature for a givenanalyte, and will translate to simpler post-measurement analysis toidentify the analyte.

The precise value of these currents will depend on how much analyteinteracts with the polymer 207. This interaction can be controlled forany given cell by adding certain receptor molecules, the interactionwith a given analyte can be increased. A given receptor molecules aredesigned to attract a certain class of chemicals. The sensing materialsare organic semiconductor polymers doped with a small molecule receptor,but can also be other types of polymers such as insulator polymers withcarbon nanoparticles. All sensor materials used are solution-based,allowing low-cost deposition methodologies such as inkjet printing.

Therefore, in accordance with the present invention, a plurality ofthese receptor-doped sensor cells, each with a different receptormolecule, or different levels of a single molecule, may be placed intoan array, as is illustrated in FIG. 3. Applied to such an array, eachcomponent of a vapor being measured will give rise to a differentsilicon and polymer current from each of the receptor-doped sensor cellsof the array, the accumulation of which make up an “electronicsignature” of the vapor. This electronic signature can then be comparedto previous measurements to identify the same vapor or components of thesame vapor. As discussed above, this vapor signature consists of twocurrents from a series of sensor cells in an array. However, thesecurrents are difficult to store and to compare to other vaporsignatures. For this reason, these signals are converted to a digitalformat. One skilled in the art would employ Analog-to-Digital (A-to-D)converters for this purpose. However, it is desired to find circuitrythat can fit on the pitch of the array so that the array itself willoutput the vapor signatures. Such a design would bring about designsimplifications that will results in significant cost reductions of thefinal product.

Referring next to FIG. 3, a method and apparatus are disclosed in whicha Voltage Controlled Oscillator (VCO) is used to convert a current levelto a frequency. As the current level increases, so does the frequency,and vice versa. A counter is the used to count the number ofoscillations per unit time. These values are then stored and used as asignature to determine a given analyte.

In FIG. 3, a sensor array 210 is provided with one or more sensorelements 2151 . . . 215 n. Each sensor element is preferably of the typedepicted in FIG. 2, but may employ any type of desired sensor design.Each sensor from the sensor array preferably provides a correspondingcurrent level output 2201 . . . 220 n related to the level of gas orother material sensed. This output relationship is preferably known foreach sensor employed. It has been determined that such current outputmay also vary over time in accordance with various characteristics ofthe sensor, including but not limited to temperature, amount of time agas has been in contact with the sensor, and the like.

Output current 220 ₁ . . . 220 _(n) from one or more of sensors 215 ₁ .. . 215 _(n) is then forwarded to corresponding Voltage ControlledOscillators (VCOs) 225 ₁ . . . 225 _(n). Such voltage controlledoscillators provide an output oscillation, the frequency of which variesdirectly in accordance with an input current. Therefore, in accordancewith the invention, as each sensor detects an amount of a gas to whichit is sensitive, the sensor outputs a related current. This current isprovided as an input to the corresponding VCO, which is then caused togenerate an oscillating output related to the level of the currentinput. As is further shown in FIG. 3, this oscillating output is in turnprovided to a corresponding counter 230 ₁ . . . 230 _(n). Each counter230 thus counts a number of oscillations output from corresponding VCO225 for a predetermined period of time. Such total counted number ofoscillations per unit time is therefore indicative of the amount of gassensed by corresponding sensor 215 over that period of time. Thesecounts can then be stored in conventional memory 235 and preferably arestored associated with various identifying data, including counteridentifier, time period identifier, time period duration, and any otherdesired information that would allow for the later processing of suchstored count data to aid in analyzing detected gas concentration.

This method and apparatus allows the cells to be placed together incompact form, and permits signal control of the cells with row-orientedand column-oriented runners through the array that connect appropriatelyto the cell, much like a memory array. The VCO/counter circuitry sits onthe periphery of the array, making the design efficient in terms of areautilized. This reduction in area may result in significant costreduction.

The cells in this array are biased in a particular manner to sense theanalyte, to let the analyte accumulate, and to clear the analyte fromthe sensor cell, also referred to as “reset”. These sensor cells arecontinuously cycled through these functions. FIG. 4 presents a timingdiagram to provide a note of the order of operation, and a generalconcept of relative timing, although such relationships in FIG. 4 areapproximate and should not be considered to be precisely accurate oreven necessarily desirably consistent during operation.

A sensing cycle in accordance with the present invention preferablyconsists of 6 steps, which are then preferably repeated each time areading from the sensor is to be taken. Performing these steps willresult in a set of output signals for each sensor cell. The combinationof these output signals can then be utilized to identify a given analytein a particular cell, or can be combined with outputs from one or moreadditional sensor cells to provide a more extensive test result to morespecifically identify one or more analytes. In each step, the bias noteddescribes the voltage level at the particular noted physical location onthe sensor cell In order to perform such an action, a bias is appliedacross the electrodes, and thus across the receptor of the sensor cellto aid in different desired movement of analyte material.

The first step, noted at timing C1 in FIG. 4, comprises a Reset Cycle.During this time period, the objective is to remove analyte from thereceptor in the sensor cell. The Bias: SS=5 SD=5 OS=0 OD=0 insures thatthere is no current movement between SS and SD (as they are bothmaintained at approximately 5V), and there is also no current movementbetween OS and OD in that they are both maintained at ground. Thesevalues, however, do generate an electric field. As analyte is generallya negatively charged particle, the analyte is driven from the highervoltage of the SS and SD layer out past the OS and OD layer and out ofthe receptor in the sensor cell. No measurements are made from thesensor cell in this step.

The second step takes place at timing C2, with an objective to obtainthe silicon current (SS-SD) at the start of analyte accumulation in asensitive part of transistor regime and to quantify any drift due tochanges in the materials used in the gas sensor cell. In particular thecurrent of S1P, (I(S1P)) is determined by measuring the current betweenthe SS and SD electrodes, so I(S1P)=I(SS-SD), to measure a siliconcurrent signature before analyte accumulation in a most sensitiveregion. The notation of “P” here is intended to indicate that thismeasurement detects the change in Vt of the silicon transistor as aresult of increased charge from the analyte, performed in a mostsensitive regime of the transistor characteristics. The electrodes arecharged so that SS=0 SD=1 OS=0 OD=0, and therefore there is a slightcurrent flow from SS to SD, and no current flow from OS to OD. There isalso no voltage gradient in the receptor of the sensor, and thereforeanalyte is neither drawn into nor expelled from the receptor. Themeasurement in this second step detects the change in Vt of the silicontransistor as a result of increased charge from the analyte, andindicator of analyte charge starting at a time when analyte movementinto the receptor of the sensor cell, just at the threshold of detectionthereof.

The third step takes place at timing C3 in FIG. 4, and in particular theobjective is to obtain a measure of organic and silicon currentsignature immediately after analyte introduction, and therefore performmeasurements of I(O1) and I(S1). In this step, the electrodes are biasedso that SS=0 SD=1 OS=5 OD=3 so that there is current flow from SD to SSand from OS to OD. Furthermore, the voltage gradient set up between theorganic electrodes (OS, OD) and the silicon electrodes (SS, SD) aids todraw analyte into the receptor. I(S1)=I(SS-SD) is measured to determinethe silicon current signature immediately after analyte introduction,and I(O1)=I(OS-OD) is measured to determine the organic currentsignature immediately after analyte introduction.

The fourth step takes place on the timing diagram at step C4, allowingfor analyte accumulation. Analyte is attracted to the organic materialdue to a positive charge on sensor material, generated in accordancewith the electrode bias that is unchanged from step C3 (SS=0 SD=1 OS=5OD=3). Thus, analyte accumulation is encouraged during this time period,and while no measurements are necessarily planned, additional I(OX) andI(SX) currents may be measured during accumulation to aid in determiningcharacteristics thereof.

After sufficient analyte accumulation, measurements of I(O2) and I(S2)are performed at a time C5 in FIG. 4 to obtain organic and siliconcurrent signature after analyte accumulation. The silicon currentmeasurement may be made simultaneously, or sequentially to the organiccurrent to prevent cross-talk of the two measurements, as desired. Onceagain, the biasing currents are unchanged from the above steps, so thatSS=0 SD=1 OS=5 OD=3. Measurement I(S1)=I(SS−SD) is made to determine thesilicon current signature after analyte accumulation, and measurementI(O1)=I(OS-OD) to determine the organic current signature after analyteaccumulation.

Finally, at step C6: a measurement of I(S2P) is made to obtain thesilicon current after analyte accumulation in most sensitive part oftransistor regime. Bias on the electrodes is changed so that SS=0 SD=1OS=0 OD=0 so that while there is a small current flow from SS to SD,there is no current flow from OS to OD, and there is no voltage gradientacross the receptor of the sensor cell. I(S2P)=I(SS-SD) is then measuredto determine the silicon current signature after analyte accumulation insensitive regime

Then, processing returns to step C1 for a next cycle of reading.Further, while this cycle is described with six steps in order, lesssteps may be employed, with a reduction in the number of measuredvalues. Further, the order of the steps may be changed, as long assufficient time is allowed between steps to allow for a steady state ofanalyte to be reached. Additionally, a reset step need not be performedeach cycle, and may be performed less than every cycle, when necessaryto clear the analyte from the sensor cell.

In a preferred embodiment, a plurality of sensor cells are provided inan array format, each preferably having different receptor portions.Thus, through the use of the above steps for multiple sensor cells, ananalyte signature can be generated to aid in determining variouscharacteristics thereof.

Therefore, in accordance with the invention, a method for performing oneor more measurements of the disclosed sensor cell, or variants thereofincluding similar electrode structure, is described. It is contemplatedthat a processor or other computing and calculation processor functionto control this process, changing the electrode currents, storing andimplementing the processing program algorithm, storing reading resultsand the like

Referring next to FIG. 5, an alternative embodiment of the invention isshown. In FIG. 5, a sensor array 310, comprises two or more sensorcolumns 312 ₁ . . . 312 _(m), each sensor column 312 comprising one ormore sensor elements 220 ₁ . . . 220 _(n), similar sensor elements tothose described in accordance with FIG. 3. Thus, an m column, n sensorarray is disclosed. Of course, the sensor elements need not bedistributed in such a regular array, and indeed may be positioned in anydesirable configuration. Each sensor 220 is then preferably connected toone or more on chip circuitry elements 340 ₁ . . . 340 _(n). It iscontemplated that each of these on chip circuitry elements contains ainput to receive a current output from one or more sensor cells 220, andfurther comprises a VCO 225 and corresponding counter 230 and memory 235as described in relation to FIG. 3.

Each chip circuitry element may also include a switching element toswitch input paths to be processed be the circuitry, thus effectivelyselecting from which sensor element 220 the circuitry is to receive asignal at a particular time. A more passive switching element may beavailable if the various output signal from the sensor elements 220 arethemselves controlled so that only one sensor element is outputting asignal to a particular circuit element 340 at a time. Furthermore, whileon chip circuitry elements are described, it is contemplated that theuse of such sensor array columns need not be so limited, and in fact theoutputs from these sensor arrays may be provided to any type of countingor analysis mechanisms.

During use, one column of the array is preferably activated at a time toallow for sensor information to be provided to on chip circuitryelements 340, much in the manner as described in accordance with FIG. 3.After reading information for a predetermined time from a particulararray column, it is then possible to switch and read further informationfrom a second of the array columns, thus increasing the speed of thedevice. Thus, while one column of the device is reading information, theothers may be resetting or be otherwise processed. For a situation wherea large number of consecutive readings must be taken, a large number ofarray columns may be provided and used sequentially. While it iscontemplated that each array column include a similar set of sensorelements, different sensitivity sensor elements may be provided to allowfor more flexibility in testing.

Possible processing that may be applied to an array column may compriseapplication of an electrical field to the sensors, or heating theelements of a particular sensor or sensor array column, for the reasonsnoted above with respect to disassociation of the analyte and thereceptor of the sensor cell. Referring next to FIGS. 6 and 7, suchapplication of an electric current and heating of individual sensorcells or columns will be described.

Dissociation of the analyte from the receptor after a given measurementis completed can be achieved with an electric field perpendicular to thefilm surface to drive the analyte out. Due to the structure of thepreferably employed inventive sensor cell, a vertical electric fieldthrough the polymer of each sensor element can be controlled. When theelectric field induces a positive charge on the polymer, the analyte isabsorbed into the polymer. When the electric field is reversed, theanalyte is repelled from the polymer. Therefore, the control of theelectric field in the sensing polymer of the cell reduces the need forvalves and piping systems to bring the analyte to and from the sensingsurface. Therefore, in accordance with the invention as set forth inFIG. 7, application of such an electric field can be applied to one ormore of the individual sensors 220, or preferably to one or more of thesensor array columns 312 to attract the analyte thereto prior to sensorreadings being made. However, in some cases the receptor binds to theanalyte with sufficient strength that the electric field does notachieve complete dissociation. In these cases, it becomes necessary toheat the cell.

Traditionally, a separate heating pad is placed underneath the sensors.However, this method is bulky, expensive, and power-consuming.Therefore, as is shown in FIG. 6, a heating mechanism provided byplacing an n+ in the vicinity each cell. When current is passed throughthis n+, resistive heating will raise the temperature of the area aroundit, thereby heating the sensor cells that are in the vicinity. Inaccordance with the invention, each of these n+ elements may becontrolled individually, or may be alternatively controlled to act inconcert for a particular array column, thus allowing the placement ofsensor elements in an array column in close proximity to each other asthey will be heated simultaneously, while allowing for the placement ofthe array columns to avoid heating of one column through the activationof the n+ heating elements associated with an adjacent sensor arraycolumn.

The benefits with this method of heating are two-fold: (1) this heatmechanism can be incorporated directly into the silicon chip, and (2)this method provides a local heat mechanism, permitting only the cellsthat require heating at any specific time to be heated.

Therefore, as is further shown in FIG. 7, one or more of the sensorarray columns may be unheated (and also un electrified as noted above toremove analyte from the receptor. It is preferred that these sensorarray columns be providing sensing information to the correspondingcircuit elements 340, or other elements for accumulating informationfrom the sensor arrays, as noted above. Other of the array columns maybe heated, and/or electrified to drive the analyte form the receptors ofthe sensor cells in these sensor array columns, thus preparing thesesensors or future sensing operations. As multiple array columns may beprovided, various of these columns will be in different states, havingbeen subject to electrical current and/or heating for different periodsof time in accordance when they were last used for a sensing operation.Therefore in accordance with the invention, sensor array columns may beelectrified and/or heated in sequence, or at a timing as desired toallow for the most efficient sensing operations to be undertaken.

In addition to preparing the sensors for future sensing operations,performing a sensing operation while a sensor element or sensor arraycolumn is being electrified and/or heated may be desirable. The use ofsuch localized heating in a reference sensor cell may allow for abaseline reading to be taken at any time, thus improving the accuracy ofthe sensing apparatus. Such use of localized heating in a reference cellor reference sensor array column may bring about several key advantages,including but not limited to (1) a continuously-updated baseline; (2) ameasure of the activation energy of the analyte to the receptor; and (3)an increase in the number of measurements per unit time.

The following process may allow for the use of one or more of the sensorarray columns to be employed as a reference sensor array column. One ormore columns of sensors is subjected to localized heat (and/orelectrification) that is generated by passing current through n+diffusion in a p-type silicon substrate (or a p+ diffusion in n-typesilicon) that is routed in the vicinity of those sensors, as describedin accordance with FIG. 6. One or more additional columns ofsubstantially identical sensors may preferably not be subjected to thisheat. A measure of the current between the source and drain off thesilicon device and the source and drain on the organic device of eachsensor is run to the periphery of the array, along the chip, andcaptured by circuitry 340. The silicon device currents of the heated andunheated devices and the organic device currents of the heated andunheated devices may then be compared to provide the above benefits,which will now be described in more depth.

At elevated temperatures, the analyte and the receptor are far lesslikely to bond. Therefore, the current generated in the heated sensorscan act as a baseline in comparison to the sensor cells at roomtemperature. In traditional sensor arrays, the baseline is generated bya measurement made by the same sensor at a previous time. In accordancewith the invention as set forth in FIG. 7, the baseline may becontinuously updated at nearly the very same time as the analyte ismeasured with the unheated sensor, giving rise to several benefits:

-   -   (1) The measurement will be more accurate since the baseline is        measured at the same time as the analyte measurement,        eliminating errors created due to the time shift between the        baseline measurement and the analyte measurement.    -   (2) automated calibration to moisture    -   (3) The analyte measurements can be taken at more frequent        intervals, or even continuously, since there is no need to stop        the sensor measurement to re-establish a baseline.

Activation energy is a measure of the temperature dependence of achemical reaction. Specifically, when ln(x) vs 1/T is plotted, the slopeof that equation is the activation energy Ea. This information can beobtained for both the silicon device and the organic device of thesensor cells. Since activation energy is specific to the type ofanalyte, this information helps significantly in identifying the analytewith reduced signal analysis.

Furthermore, additional benefits of selectively heating one or more ofthe sensor cells is that the Polymer sensors preferably employed inaccordance with the present invention are known to pick up moisture overtime, causing the base signal for any given analyte to drift. Thisproblem is generally resolved by heating the cell, thereby driving awaymoisture. Thus, each heated column is not susceptible to being affectedby any such moisture accumulation since it is being heated. However, asmoisture accumulates in the polymer the baseline signal of the unheatedcell could drift. Thus, as described above, through the selectiveheating of individual sensor cells or sensor array columns, and inparticular through the interchanging of heated and unheated sensor arraycolumns, such drift issues may be eliminated.

In particular, in order to avoid such baseline drift, two or moresubstantially identical columns of sensor arrays are placed in an array,each with a capability of heating. In a first cycle, one or more of thecolumns are heated and others of the one or more columns are unheated.In a subsequent cycle, one or more of the previously unheated columnsare heated, and one or more of the previously heated columns areunheated. Operation is altered between two or more such cycles at afrequency necessary to keep moisture levels in the cell sufficiently lowto avoid baseline drift.

Therefore, in accordance with the invention, a sensor array constructedin accordance with one or more of the embodiments of the invention maybe integrated into a standard CMOS silicon process, thus providing oneor more of the following benefits over prior art sensor products.

Manufacturability:

The sensor cell can be integrated directly into CMOS silicon, and thesensing polymers are solution based allowing low cost high volumedeposition methods.

Increased Signal Information from Each Sensor Cell:

Signals convey information about the analyte's charge (silicon current),size (polymer current), and activation energy (comparison betweensignals at two different temperatures). This increased information mayresult in better analyte selectivity and reduced need for signalanalysis.

Control Over Analyte/Receptor Interaction:

The cell structure allows the charge of the polymer to be controlled byan electric field, attracting or repelling the analyte from the sensormeasurement surface.

Increased Immunity from Sensor Drift:

The measured sensor cell is compared to the same cell at a hightemperature, giving rise to a continuously updated baseline signal,increasing immunity to sensor drift.

Moisture Resistance:

One of two identical columns of sensor cells are heated while the othercolumn is being measured, and are swapped with sufficient frequency sothat moisture build-up in either column of cells becomes a problem.

Receptor Diversity:

In addition to organic semiconductor polymers doped with a receptor, thecell structure can accommodate a wide variety of different polymer-basedsensing materials.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,because certain changes may be made in carrying out the above method andin the construction(s) set forth without departing from the spirit andscope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that this description is intended to coverall of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall there between.

What is claimed:
 1. An apparatus for storing sensor information,comprising: a plurality of sensors in an array for sensing one or moreparameters by, each sensor providing an output current in accordancewith a level of the sensed parameter; a plurality of Voltage ControlledOscillators (VCOs) in an array, each such VCO corresponding to one ofthe plurality of sensors for receiving the output current and generatingan output oscillation frequency in accordance with the level of thereceived output current; and a plurality of counters in an array, eachsuch counter corresponding to one of the VCOs, for counting a number ofoscillations over a predetermined time.
 2. The apparatus of claim 1,wherein the parameter comprises a level of an analyte coming intocontact with a receptor of the sensor.
 3. The apparatus of claim 1,further comprising a memory for storing the counted number ofoscillations and one or more identifying characteristics of the countednumber of oscillations.
 4. The apparatus of claim 1, wherein the arrayof sensor, VCO and counter elements are provided on a single CMOSsilicon chip.
 5. The apparatus of claim 1, wherein each sensor elementsincludes a different receptor composition.
 6. The apparatus of claim 1,wherein the sensor comprises a 4 T Cell sensor.
 7. An apparatus forsensing analyte, comprising: a plurality of sensor array columns, eachsensor array column comprising a plurality of sensors, each sensor beingadapted for sensing one or more parameters in reaction to the presenceof one or more analytes and output a current therefrom in accordancewith level of the sensed parameter; a Voltage Controlled Oscillator(VCO) array comprising a plurality of VCOs, each VCO adapted to receivean output current from one of the plurality of sensors in a first of theplurality of sensor arrays and to receive an output current from one ofthe plurality of sensors in a second of the plurality of sensor arraysand for and generating an output oscillation frequency in accordancewith the level of the received output current from the correspondingsensor in the first sensor array and the corresponding sensor in thesecond sensor array; and a counter array comprising a plurality ofcounters, each counter adapted to receive an output from a correspondingVCO and count a number of oscillations over a predetermined time.
 8. Theapparatus of claim 7, wherein the level of the sensed parametercomprises a level of an analyte coming into contact with a receptor ofthe sensor.
 9. The apparatus of claim 7, further comprising a memoryarray, the memory array comprising a plurality of memory locations, eachmemory location being adapted to store a counted number of oscillationsand one or more identifying characteristics of the counted number ofoscillations from each of a corresponding counter.
 10. The apparatus ofclaim 7, wherein the array of sensors, VCO array and counter array areprovided on a single CMOS silicon chip.
 11. The apparatus of claim 7,wherein each sensor element in a sensor array includes a differentreceptor composition.
 12. The apparatus of claim 7, wherein each sensorarray includes sensors having a substantially similar group of differentreceptor compositions.
 13. The apparatus of claim 7, wherein the sensorcomprises a 4 T Cell sensor.
 14. An apparatus for sensing analyte,comprising: a plurality of sensor array columns, each sensor arraycolumn comprising a plurality of sensors, each sensor being adapted forsensing one or more parameters in reaction to the presence of one ormore analytes and output a current therefrom in accordance with level ofthe sensed parameter; and a switching mechanism for selectivelyswitching one or more of the plurality of sensor array columns on tosense the one or more parameters and to output a current from each ofthe sensor cells thereof, and for selectively switching one or more ofthe plurality of sensor arrays off; a corresponding Voltage ControlledOscillator (VCO) arranged in a VCO array receiving an output currentfrom each of the plurality of sensors in a first of the plurality ofsensor array columns, receiving an output current from each of theplurality of sensors in a second of the plurality of sensor arraycolumns by the corresponding VCO arranged in the VCO array, andgenerating an output oscillation frequency by each VCO in accordancewith the level of the received output current from the correspondingsensor in the first sensor array column and the corresponding sensor inthe second sensor array column; and a plurality of counters arranged ina counter array, each such counter corresponding to one of the pluralityof VCOs, for counting a number of oscillations generated at each of theVCOs over a predetermined time.